1. Field of the Invention
The present invention relates to planographic printing plate precursors, which has sensitivity to ultraviolet light rays, visible light rays or infrared light. More particularly, the present invention relates to a negative image recording material which can perform so-called “direct printing plate production” and can directly make a printing plate using a laser beam in accordance with a digital signal outputted from a computer or the like.
2. Description of the Related Art
Laser technology has remarkably advanced in recent years with widespread availability of high output and small-sized lasers utilizing ultraviolet rays, a visible light rays and infrared rays. These lasers are very useful as a light source for recording when printing plate making is performed by direct printing plate production in accordance with the digital data outputted from computers and the like (hereinafter, this direct printing plate production is referred to also as computer to plate, or CTP in short). Particularly, since solid state lasers and semiconductor lasers, which emit infrared rays having a wavelength of from 760 nm to 1200 nm, have high output compared with other wavelength regions, these lasers are very useful. Accordingly, in recent years these has been an increased demand for an image recording material which has a high sensitivity to such an infrared ray laser, that is, the image recording material, solubility in a developer of which is greatly changed by irradiation with the infrared ray.
In a negative image recording layer in such an the planographic printing plate precursor, there is a problem that, in one type which is cured by being exposed to light causing a polymerization reaction to proceed, the image recording layer has a relatively low strength before being cured and, in another type which contains a polymerizable low molecular weight compound, a support and the image recording layer adhere to each other easily; therefore, in either case, there is a problem that the image recording layer is easily damaged.
In a common writing method which utilizes visible light, an image forming reaction accurs, which utilizes light for either a laser exposure or a lamp exposure whereupon sufficient photoreaction occurs in the image recording layer up to a deep portion of the image recording layer in the vicinity of the support. Therefore, even when a scratch mark is formed only on a surface of the image recording layer, an image blank area is not generated. Only when a large scratch mark is formed, which reaches the deep portion, is an image defect such as the image blank area generated.
On the other hand, in a writing method of the invention, which utilizes such an infrared ray laser, image forming is performed by converting the infrared ray into heat and increasing a temperature of the image recording layer. In this method, particularly when a general support such as aluminum is used, due to thermal diffusion to the support having a high thermal conductivity, a sufficient temperature elevation cannot be achieved in the deep portion of the image recording layer while a sufficient temperature elevation can be achieved in the vicinity of a surface of the image recording layer whereupon a thermal reaction is efficiently performed. In a case of such an image forming mechanism, even when a shallow scratch mark is formed only in the vicinity of the surface of the image recording layer, since a curing reaction occurs and, further, a scratch mark is formed in a curing reaction region in the vicinity of the surface, which region will become a developing agent resistant region, the image blank area easily generated. Accordingly, an image forming material, to which a recording method using an infrared ray as in the invention is applied, is easily effected by a scratch mark such that image quality is easily deteriorated by the scratch mark, as compared with the writing method using visible light and the like.
Further, by present inventors found that, in a developing process for removing an unexposed portion by using an aqueous alkaline solution and performed after recording by light exposure, a decrease of film thickness in a region of an image portion clearly occured even though the decrease was small. It is assumed that, on the surface of the image recording layer in which image forming is most efficiently performed by polymerizarion reaction, a polymerization inhibition is generated by oxygen present in the atmosphere whereupon a portion in the vicinity of the surface, which portion is in sufficiently cured is generated and, accordingly, this insufficiently cured portion is dissolved and removed by the aqueous alkaline solution.
To prevent this image portion defect caused by a scratch mark and decrease of film thickness by poor curing, it is effective to provide a surface protective layer using a water-soluble resin having a low oxygen permeability or the like; however, when the protective layer is left intact on the surface of the image portion, since such a protective layer adversely affects an ink accepting property and, is also hydrophilic, there is a fear that stickiness will be generated on the surface in a storage environment, thereby deteriorating a working property.